Carbon dioxide degassing at Latera caldera (Italy): evidence of geothermal reservoir and evaluation of its potential energy.

In order to test the potentiality of soil CO2 diffuse degassing measurements for the study of underground mass and heat transfer in geothermal systems detailed surveys were performed at Latera caldera, which is an excellent test site, because of the abundant available subsurface data. Over 2500 measurements of soil CO2 flux revealed that endogenous CO2 at Latera caldera concentrates on a NE-SW band coinciding with a structural high of fractured Mesozoic limestones hosting a water-dominated high-enthalpy geothermal reservoir. The total hydrothermal CO2 degassing from the structural high has been evaluated at 350 t d(-1) from an area of 3.1 km(2). It has been estimated that such a CO2 release would imply a geothermal liquid flux of 263 kg s(-1), with a heat release of 239 MW. The chemical and isotopic composition of the gas indicates a provenance from the geothermal reservoir and that CO2 is partly originated by thermal metamorphic decarbonation in the hottest deepest parts of the system and partly has a likely mantle origin. The ratios of CO2, H-2, CH4, and CO to Ar were used to estimate the T-P conditions of the reservoir. Results cluster at T similar to 200-300 degrees C and P-CO2 similar to 100-200 bars, close to the actual well measurements. Finally, the approach proved to be an excellent tool to investigate the presence of an active geothermal reservoir at depth and that the H-2-CO2CH4-CO-Ar gas composition is a useful T-P geochemical indicator for such CO2 rich geothermal systems.